1. Field of the Invention
This invention relates to thermoplastic partially crystalline polyester coating powders particularly useful in fluidized bed sintering processes, electrostatic powder coating and electrostatic fluidized bed processes. More particularly, this invention is directed to an improvement in powder coating process wherein a polyester powder is applied to a substrate. This invention is particularly directed to obtaining polyester coatings on substrates which polyester coatings are applied to the substrate by a powder coating process wherein the resultant coating cannot be loosened from the substrate by a knife, the coating exhibits good Erichsen elasticity and has a good appearance. More especially this invention is directed to partially crystalline thermoplastic polyester coating powders having good stability in storage.
2. Discussion of the Prior Art
It is known to produce coatings by a fluidized bed process or by the electrostatic powdering method, using thermoplastic polyesters. Either amorphous types of copolyester are used or highly crystalline polyesters such as those formed from polyethylene terephthalate (PETP).
Amorphous polyesters or coating powders made therefrom in many respects having outstanding adhesive characteristics with respect to many metals. They can be used in fluidized bed coating processes wherein they are deposited on such metal substrates. They are used advantageously on such substrates on account of their weather-resistant properties. A serious disadvantage has been found in the use of such amorphous types of copolyesters. They are characterized by poor stability in storage. Additionally, the elasticity of the resultant coatings is often inadequate, especially where the coating has a thickness greater than 150 microns, such as those coatings produced by the fluid bed method.
During storage and transportation it has been additionally found that such coating powders tend to become lumpy so that they are no longer practically usable. In addition, the dipping of hot parts into such powders heats the vats employed in the fluidized bed process which contain the polyester powders to such an extent that such polyester powders stick to the walls and to the porous bottom. Pieces which are coated with such powders are not resistant to hot water. The inadequate elasticity of the coatings in the greater thicknesses is evidenced by crazing under mechanical stress and by poor Erichsen cupping and impact indentation characteristics. Many efforts have been made to provide suitable coating powders from polyesters derived from polyethylene terephthalic, polytetramethylene terephthalic and other highly crystalline polyesters. Experiments have shown that the stability in storage of such coating powders is excellent. These polyesters have the disadvantage, however, that the elasticity of the coatings and the adhesion characteristics are poor, and especially on smooth metal surfaces. The tensions which may occur within the coatings are in some cases so great that even mild mechanical stress, such as when coated metal substrates are bent, results in cracking. The coated films can then readily be lifted from the metal substrates surface.
The mechanical characteristics and the adhesive properties of such coatings cannot be improved sufficiently by quenching the hot pieces immediately after coating. If highly crystalline polyesters of great molecular weight are employed an improvement of mechanical properties is achieved but the appearance of the coated metal parts suffers due to poor leveling. Overall this is a very negative effect.
There is thus a desire to provide suitable coating powders of thermoplastic polyesters. Thus far thermoplastic polyesters have not achieved any great economical importance owing to the aforenoted disadvantages. It is an object of the present invention, therefore, to provide polyester coating powders which do not suffer from the above-described disadvantages, provide coatings which are characterized by good elasticity and bending properties, are stable in storage, provide coatings having good appearance and are otherwise suited for coating by known powder coating procedures. It is a further object of the present invention to provide such polyester coating powders which will readily adhere even to smooth metal surfaces such that they do not become delaminated.